WASHINGTON (CNN) --Some White House staffers were interviewed late Thursday by the FBI as part of investigation into who leaked to the media the identity of a CIA operative, government sources told CNN. Interviews were expected to continue Friday, these sources said.

The sources would not say which officials have been interviewed, but they said White House senior adviser Karl Rove has not yet been questioned.

Former Ambassador Joseph Wilson had once said Rove was responsible for the leaking of his wife's CIA employment to syndicated columnist Robert Novak. Wilson later backed off that assertion, but still said Rove might have condoned the leak. Through a White House spokesman, Rove has denied any role in the leaking of that information.

In his July 14 column, Novak said the information came from senior administration officials.

Wilson has charged his wife's identity was leaked as a punitive move, noting that it followed the publication of an op-ed article Wilson had written that criticized the administration handling of pre-war intelligence on Iraq. White House officials have disputed that characterization and President Bush has vowed full cooperation in the probe.

Some CIA officials had been interviewed earlier as part of the probe.

Sources told CNN no officials at the State or Defense departments have been interviewed.

Meanwhile, four leading Democratic senators accused the White House on Thursday of bungling the investigation.

In a letter to Bush, Minority Leader Tom Daschle of South Dakota and three others said the White House has made "at least five serious missteps" in the leak probe so far.

The biggest is leaving Attorney General John Ashcroft in charge of the probe rather than naming a special prosecutor to handle it, as they have called for previously, they said.

"We are at risk of seeing this investigation so compromised that those responsible for this national security breach will never be identified and prosecuted," the senators wrote.

The letter was signed by Sens. Joseph Biden of Delaware, ranking Democrat on the Senate Foreign Relations Committee; Carl Levin of Michigan, ranking Democrat on the Armed Services Committee; and Sen. Charles Schumer of New York, one of the first lawmakers to call for an investigation of the leak.

Bush has repeatedly said he believe the Justice Department can properly handle the probe.